The invention relates to electrical engineering, in particular, to capacitor-making industry and can find application in producing high-capacity electric capacitors making use of double electric layer (DEL). Capacitors with DEL have found application as stand-by power sources in systems requiring uninterrupted power supply, such as computation engineering, communications equipment, numerically controlled machine tools, in uninterrupted cycle production processes; for electric-starter starting of diesel engines; for power supply of invalid carriages, golf carriages, and so on.
Known in the art presently are electric energy accumulators appearing double electric layer (DEL) capacitors, e.g., those disclosed in U.S. Pat. No. 4,313,084 (1982) and U.S. Pat. No. 4,562,511 (1985). Said capacitors comprise each two porous polarizable electrodes and porous separator made of a dielectric material and placed therebetween, and current leads. A liquid electrolyte solution in the capacity of which either aqueous or non aqueous electrolytes (aqueous sulfuric acid inclusive) are used, is contained in the pores of the electrodes and separator, as well in a certain free space inside the capacitor casing. Electric charge is accumulated at the interface in the pores between the electrode material and the electrolyte. Used as the materials of polarizable electrodes are various routine porous carbon materials. To increase the capacitor capacitance with double electric layer said carbon materials are subjected to preactivation with a view to increasing their specific surface area up to 300-3000 sg.m/g.
DEL capacitors possess much higher capacitance compared with routine film-type and electrolytic capacitors, amounting to a few scores farads per gram of active electrode materials. However, said capacitors suffer from the disadvantage that they have but rather low specific energy, i.e., as low as 3 W-h/lit. In this case maximum specific energy values are attainable with dual-layer capacitors using non-aqueous electrolytes for which the maximum voltage is within 3 and 3.5 V. However, such capacitors allow very low charging and discharge current values to be obtained, which is due to very low conductivity values of non-aqueous electrolytes. For dual-layer capacitors using aqueous electrolytes featuring maximum voltage value of 0.8 V still lower specific energy values are attainable, i.e., 0.5 to 2 W-h/lit. When such dual-layer capacitors are in a charged state for a rather prolonged period of time with voltage exceeding 0.8 V, noticeable oxidation of the positive carbon electrode occurs.
Closest to the proposed invention as to the technical essence and attainable effect is a DEL capacitor comprising two electrodes and liquid electrolyte (i.e., an aqueous alkali-metal hydroxide with a concentration of 3 to 7 mole/lit having a polarizable (negative) electrode made of a carbon fibrous material and a non-polarizable electrode made of nickel oxide. Maximum voltage of said capacitor equals 1.4 V and specific capacitance and specific energy, 46 F/cu.cm and 45 J/cu.cm, respectively (WO 97/07518 dated Feb. 27, 1997).
However, the capacitor in question suffers from a number of disadvantages such as inadequately high specific energy and high cost accounted for by use of large amounts of nickel oxide.
It is an object of the present invention to provide a DEL capacitor featuring a higher specific energy.
It is another object of the invention to reduce the production cost of the capacitor involved. The foregoing objects are accomplished due to the invention disclosed hereinbelow the essence of which resides in that the capacitor in question has a polarizable electrode made from a porous carbon material and a non-polarizable electrode made from a material comprising lead sulfate as an active component, as well as in that used as electrolyte is an aqueous solution containing sulfuric acid.
It is preferable that the capacitor incorporate a current lead provided with a protective coating made from graphite foil impregnated with an acid-resistant polymer.
It is expedient that the capacitor also incorporate two polarizable electrodes, one non-polarizable electrode, and two separators, all of said component being arranged in the following order: the first polarizable electrode/the first separator/the non-polarizable electrode/the second separator/the second polarizable electrode, both of the negative electrodes being short-circuited on each other. With such an arrangement of the electrodes and separators the specific capacitance of the polarizable (negative) electrode is substantially lower than that of the non-polarizable (positive) electrode, whereby a total thickness of the negative electrode is much larger than that of the positive electrode. Hence the herein-proposed splitting of one negative electrode into two electrodes having half-thickness each provides for virtually one-half reduction of ohmic loss of energy with rather high current density values.
It is also expedient that the material of one or all electrodes is doped with a particulate material, such as polytetrafluoroethylene or polyethylene. First, this makes it possible to produce the negative electrode from not only a fibrous carbon material (e.g., a carbon fabric which is the case with the known capacitor (cf. WO application # 97/07,518) but also from a material based on carbon powders with the use of a polymer binder. The latter electrode is much cheaper one. Secondly, use of a polymer binder makes possible attaining an increased strength of both the negative electrode and the positive (lead-sulfate) one.
It is expedient that one capacitor or a bank of capacitor elements be compressed between the load-bearing covers of the casing so as to considerably reduce the capacitor internal resistance, especially when using negative electrodes from carbon fabric or felt, and to prevent the active material of the positive electrode from shedding which is one of the principal causes that restrict cyclicity of the DEL capacitors made according to WO application # 971/07,518. It is due to the above specified technical solutions that it is possible to essentially increase the specific energy and to reduce the cost of capacitors. An increase in the specific energy up to 2.0 V is provided due to a 1.5-times increase in the electric conduction of the sulfuric-acid electrolyte compared with the alkaline one. The cost of a DEL capacitor made in accordance with the present invention is decreased due to the use of a lead-sulfate electrode which is much cheaper than a nickel-oxide one. Use of lead sulfate as the active material of the positive electrode provides a possibility of using diluted sulfuric acid as electrolyte which facilitates much the assembling of the DEL capacitors involved.